Transmission system



'15 pling ing terminal of each windin I nected through a condenser to t e' other sides Patented Apr. as, 1927-. '1 V v ,UNITED' STATES 1,625,840 PATENT OFFICE.

HOMO] WHI'1TLE, NEW YORK, N. 1., ASSIGNOR '10 WESTERN ELEGTZIC OOIPLNY,

' INCORPORATED, 01' NEW YOBK, N. Y., A CORPORATION 01' NEW YORK.

Transmission SYSTEM.

Application use any is,

This invention relates totransinission circuits and more particularly it relates to couplin means for line sections of different c aracteristic impedances.

'6 7 As is well known, the maximum transfer of energy between two interconnected impedances takes place when the impedance of one of the elementsmatches the impedance of the other. Whenever these imped- 10 ances are unequal transition or reflection losses take place which cause a considerable reduction in the effective transmission 'therebetween.

In accordance with this invention, coumeans for two une ual im edances are rovided, by means 0 whic transition lbsses are substantially eliminated over a wide range. of frequency, even where the ratio of the two impedances varies with the Q0 frequency. In the case where the two innpedances constitute signal transmission lines of different characteristic im edances, a-

@ telephone cable and an open wire line, for example, the preferred form of coupling 86 means comprises a two-winding transformer havin one terminal'cf one winding connecte to one side of the cable, one terminal of the second winding connected to one side ofthe open wire line, and the remainbeing conof the cable and open wire line; It willbe hereinafter shown that such a coup ing means may be so designed that for high. quencies the transformer constitutes the 1 'inain-coupling network, that for low frechoice, the cou ling meansfacts as if the us were direct y connected without any shunting devices, while for. intermediate uencies, the couphng is due to both the con enser and the transformer. This invention will be better understood by' tothe following detailed'detaken. in connection with accompanying drawing,..in whichFig. lilluatrates a con meansfor two transmission lines 0 di t characteristic impedancu' Fig. 2' illustrates a con ling-means ofthis inveintion which is ced with sections respect-'to'both sides 911 line, and Fig. 8

the egtentto which reflection losses, aneeliminatedbythisinventlon. q .1 illustratesthe' A. of meansqprovid .iirpcco ance wi thisinventim range 0 .tion loss occurs for those frequencies em- 1821. Serial No. 485,447.

the characteristic im edance of an open wire line is-a proximate y constant over a wide i'rcqucncies and for illustrative Purposes, it may be assumed that-open wire a me 11 has a characteristic impedance of approximately 600 ohms for frequencies from 135 cycles to 30,000'cycles. The characteristic impedance of a cable, however, varies considerably with frequency and in the case of an unloaded #13 gauge cable for 30,000 cycles has a characteristic impedance of the order of 140 ohms, for speech frequencies such as 800 cycles, has an im edance of the order of 230 ohms, and for ringing current frequencies such as 135 cycles, has an impedance of the order of 600 ohms. Due to the wide variation in the characteristic impedance ofjthe telephonecable 10 with frequcncy, it follows that transition or reflec-- tion losses would occur if the cable were dircctly'connected to;the open wire line which has a characteristic impedance substantially independent of frequency. Since the two line sections would :present the reatest unbalance for frequencies of the or er of 30,000 cycles, it follows that the greatest transi- 8 loyed in,carrier telephony. Curve 12 in iig. 3 shows to what .extent transition losses would occur for frequencies from cycles to 30,000 cycles, if the line sections 10 and 11 were directly connected. In this figure, the abscissae re resent the various frequencies transmitte .between the two line sections while the ordinates represent the number of efl'ective'miles of standard cable at 800 cycles of transition loss incurred. The '95 horizontal base line in' Fig'. '3 therefore represents such a connection of the two lines that no transitionloss takes place for any frequency, which would be the caseif the two line sections were connected-together by an ideal transformer for each frequency I desired to be transmitted. Since from the w values amumed above, the open wire line and thecable are of the same characteristic impedance for135 cycles, curve 12 reaches the zero transition loss lineat 'a frequency" value corresponding; to cles. transition loss'with ghe limlalsh' y nested, increases rapi 1y wit t e frequ' until, as shown, by cur-V012, the 1 loss for frequencies from 25,000 to 30,000 cycles is of. the order of two and one half miles of standard gauge cable. This loss of two and one half miles for carrier fre-' quencies is quite objectionable and it is there-.

fore desirable to avoid this direct connection whenever possible.

In accordance with this invention, it has been found possible to substantially eliminate transition losses between a telephone cable and an open wire line by such a coupling means as shown in Fig. 1, wherein the two line sectionsare shown coupled by a transformer 13 and a condenser 14. Wind ing 15 of transformer 13 has one terminal connected to one side of the cable while the other terminal is connected to condenser 14. \Vinding 16 has one terminal connected to one side of line 11 and has its other terminal connected to condenser 14. The other terminal of condenser 14 is connected to both the cable and the open wire line. It has-been found that when the impedences of the cable and the open wire line are as mentioned above, winding 15 should have an inductance of approximately .0114 henrys, winding 16 should have an inductance of .052 henrys, and condenser 14 should have a capacity of .6 of a microfarad in the case where .windings 15 and 16 are series aiding. With the two line sections connected by a transformer and a condenser of such values, the reflection loss incurred is as represented by curve 17 of Fig. 3, which indicates substan-j tiall no reflection losses from 30,000 cycles unti the order of 1,000 cycles, while for 135 cycles, it has a reflection loss of 1.2 miles of standard cable.v This reflection loss for ringing current frequencies ,however, is not serious and may be more readily overcome than'the two and one half miles loss obtained for the carrier frequencies by a direct connection. For carrier frequencies say from 10,000 to 30,000 cycles, condenser 14 is of negligible impedence so that transformer 13 acts as if it were directly connected across the line. For these frequencies, the impedance seen by lookin through transformer 13 'in the direction 0 line 11, will be of the order of 140 ohms, while the impedance looking through transformers 13 in the di- 'rection of the cable will be of the order of 600 ohms. For, essential speech frequencies, however, 800- cycles for example, the condenser 14 will ofier appreciable impedance to the currents so that part of the speech frequency currents will flow seriallythrough windings 15 and 16, the remaining part flowing through condenser 14. This increase in the impedance of condenser 14 will cause the impedance across the points 20 and 21, looking through the couphng means to the open wire line, to be of the order of 230 ohms. For Very low frequencies, such as 135 cycles, condenser 14 has such a high impedance that retard coil of low impedance at that re-.

quency and consequently will generally produce some loss in transmission. It will be apparent that due to the fact that the retard coil tends to neutralize the capacity reactance of the cable the loss at 135 cycles occasioned by the introduction of the retard will be less than if theretard were inserted in a simple resistance circuit. In a particular case it may be possible to so design the retard coil with respect to the capacit of thecable that one is completely neutra ized bythe other, in which case there would actually be a gain in transmission between the cable and open wire line over the case'where the two are connected by an ideal transformer.

The two coils of transformer 13 should preferably be connected in series aiding relation. This is because of the fact that the mutual inductance between the two transformer windings is equivalent to a simple inductance in shunt to the line and in series with condenser 14 When these two transformer windings are in series aiding relation, the simple inductance element presents a negative reactanoe in this path and consequently will never combine with the nega- .tive reactance of the capacity to form a series resonant path of practically no impedance for a frequency dependent upon their values. Such a resonant shunt would be objectionable in most cases since its resonating effect would be liable to occur at some frequency within the band of fr equer 1- cie's over which uniform transmission is desired.

In man cases it will be found desirable to have t e cou ling means of the nvention so arrange as to be balanced withrespect to each side of the transmission line. In such cases, the coupling means may take the form shown in Fig. 2, 1n which two separate transformers 25 and 26 are provided on opposite sides of a condenser 27 The transformers 25' and 26 comprise toroidal core members 28 and 29, and windings 30, 31, and 32, 33 respectively. As described'above, it is preferable that windings 30 and 31, and windings 32 and 33 should be so wound on the toroidal cores that they are in series aid- 1.5 this invention in adapting the cou ling means to the interconnection of impe ance devices of various types and difierent characteristic im edances.

What is c aimed is: A

1. In a wave transmission system for transmitting signaling currents in the speech frequency range and ultra speech frequency range, two lines of different characteristic impedances, coupling means for said lines comprising a transformer for transmitting both speech and ultra speech frequencies, and means cooperating with said transformer for preventing substantial reflection losses for the whole range of frequencies. Q 2. In a wave transmission system for transmitting signaling currents in the speech frequency ran e andsignaling currents in the'ultra speec frequency range, two lines of difierent characteristic impedances, coupling 'nieans for said lines comprising a transformer having such an impedance ratio as to match the impedances of said line for frequencies in said last range for transmitlugs 30 to 33 were on the same core, telegraphsignaling currents in the 'anc'es being inductivel ductances and sa1d con enser bel'ng proportioned to have an impedance ratio var ng ting both speech and ultra speech frequencies, and means (:00 crating with said transformer for preventing substantial reflection losses for currents of the whole range of frequencies.

3. Coupling means for two lines of difierent characteristic impedances v, comprising a condenser in shunt to said lines of high imedanc-e to speech frequencies and of low impedance t6 ultra-audio frequencies, a ma netic core, an inductance in series with" eac line section and having a common terminal connected to said condenser, said inductances bein wound on said core in series aidin relation and havzin such values that s eec currents pass seria y through said win ings to a substantial degree while ultra-audio frequencies are transmitted between said line sections substantially entirely by virtue of the inductive coupling between said wind- *ings.

1 4. two line-sections for transmitting a plurality of ranges of signaling current sald sections having different frequency-impedance characteristics, coupling means for said line sections comprising a condenser in shunt'to said line sections and an inductance in series with each line and having a common terminal connected to said condenser, said inductcoupled, said inwith frequency such as to substantially match the impedances of said sections for all ofasaid ranges. s

my name this 15th dag of'July A. D., 1921. v I H RACE WHITTLE.

In a transmission system comprising In witness whereof, I hereunto subscribe 

